Cell Biology Quiz: Membrane Structure and Function

Human Organization- The Cell (I)

• All living things are composed of one or more cells.
• Cells are the fundamental units of life.
• The human body contains many different types of cells.
• Cells share common characteristics: they are surrounded by a cell membrane, they breathe, reproduce, and grow.
• Cells also differ in function, shape, and size.
  • Example: nerve cells are different from kidney cells.

Cell Shapes and Sizes

• Cells can be various shapes including rounded, oval, flat, cubical, columnar, spindle, or fusiform.
• Cell sizes vary greatly.
  • Lymphocytes are among the smallest (6µm).
  • Fat and ovum cells are among the largest (160µm).

Cell Types

• Cells are categorized into prokaryotic and eukaryotic cells.

  • Prokaryotic cells:

    • Lack a nuclear membrane and other membrane-bound organelles in their cytoplasm.
    • Surrounded by a cell wall.
    • Considered "primitive cells," exemplified by bacteria and blue-green algae.

  • Eukaryotic cells:

    • Possess a nuclear membrane and membrane-bound organelles in their cytoplasm.
    • Surrounded by a cell membrane (plasma membrane).
    • Examples include plant and animal cells.

Eukaryotic Cell Characteristics

• a) Cell (plasma) membrane: Forms the outer boundary of the cell.
• b) Protoplasm: A fluid-like material filling the cell.
  • Cytoplasm: The protoplasm outside the nucleus.
  • Nucleoplasm: The protoplasm inside the nucleus (nucleolus).
  • Includes organelles, inclusions, and other substances like water (75%), proteins, carbohydrates, lipids, nucleic acids, and minerals.
• c) Nucleus: The control center of the cell, containing the cell's genetic material (DNA).

Cell Structure

• Cell membrane (plasma membrane): A thin membrane (7.5-10 nanometers thick) surrounding the cell, separating the intracellular from the extracellular environment.
• Protoplasm: The living substance within the cell which includes the cytoplasm and the nucleus.
• Cytoplasm: The fluid-like substance outside the nucleus that contains various organelles.
• Nucleus: The central part of the cell containing DNA and other components.
  • Nucleoplasm: The substance within the nucleus
  • Nucleolus: A prominent structure within the nucleoplasm.

Cell Membrane (Plasma Membrane) Composition

• Bilayer of phospholipids (lipid bilayer): The fundamental structure of the membrane.
• Proteins: Embedded within and attached to the lipid bilayer, with diverse functions.
• Cholesterol: Modulates membrane fluidity.
• Short chains of sugars (glycoprotein and glycolipids): Enable cell-cell recognition.

Molecular Structure of the Cell Membrane

• Phospholipid bilayer acts as a barrier.
• Hydrophilic heads face outward towards the extracellular environment.
• Hydrophobic tails face inward, away from the water.
• Integral and peripheral proteins are embedded within or attached to the layer.
  • Some span the entire membrane (transmembrane)
  • Other only embedded in one side (monotopic)
  • Peripheral proteins attached to the surface.

Cell Membrane Lipids

• Phospholipids have polar heads and nonpolar tails.
• Heads are hydrophilic (water-loving )
• Tails are hydrophobic (water-fearing)
• Cholesterol molecules are embedded in the membrane lipid bilayer.

Cell Membrane Proteins

• Integral proteins:
  • Often transmembrane: span the entire membrane.
  • Examples, ion channels.
  • Monotopic, embedded in one side only. Often enzymes.
• Peripheral proteins: not embedded in the membrane; loosely attached to the surface. May act as enzymes

Functions of membrane proteins

• Structure support
• Transport of molecules.
• Cell-cell recognition
• Receptors for hormones and antigens
• Enzymatic control of chemical reactions

Cell Membrane Carbohydrates

• Attached to membrane proteins (glycoproteins) and lipids (glycolipids).
• Form the glycocalyx
• Functions: cell-adhesion, cell-cell recognition

Transport Across the Cell Membrane

• Passive Transport: Does not require energy.
  • Simple diffusion: Movement of small, nonpolar molecules from high to low concentration across the membrane.
  • Osmosis: Movement of water across a selectively permeable membrane from high to low water concentration.
• Active Transport: Requires cellular energy (ATP).
  • Moves molecules against their concentration gradient. Involves protein pumps.

Bulk Transport

• Endocytosis: Uptake of large particles into the cell: this includes phagocytosis and pinocytosis and receptor-mediated endocytosis.
• Exocytosis: Release of molecules out of the cell (e.g., secretion of proteins).

Solutions Related to Osmosis

• Isotonic: Same concentration of solutes inside and outside the cell.
• Hypertonic: Higher concentration of solutes outside the cell.
• Hypotonic: Lower concentration of solutes outside the cell.

Test your knowledge on the cell membrane's structure and functions with this quiz! Explore key components like glycoproteins, cholesterol, and the lipid bilayer. Understand the differences between prokaryotic and eukaryotic cells, as well as their unique features.